The present application relates to pneumatic, power-assisted systems, and more particularly, relates to vacuum boosters that use vacuum and atmospheric pressure to provide power-assist to a master cylinder of a fluid brake system.
Conventional automotive vacuum boosters are well-known for providing power-assist for braking a motor vehicle. Such vacuum boosters include a control volume partitioned by at least one movable diaphragm attached to a power piston. When an operator depresses the brake pedal, thereby applying an input force via a linkage to an input rod of the vacuum booster, a valve system actuates to create a large pressure differential across each diaphragm, thus forcing the power piston to move from a rest position to an apply position. The power piston pushes against an output rod that applies an output force many times the input force to the master brake cylinder of the braking system, which in turn actuates the brakes of the vehicle wheels.
Conventional automotive vacuum boosters have return springs to bias the power piston to the rest (rearward) position and to ensure that the vacuum booster does not self apply and create brake drag. When the operator applies a pedal force to brake the vehicle, a component of the pedal force is necessary to compress the return spring. Compressing the return spring does not contribute to braking the vehicle and therefore is a direct force loss to the brake system. This force loss is especially significant when the operator needs to brake when the engine of the vehicle has stopped running, and vacuum is no longer supplied to the vacuum booster. The pedal force required to compress the return spring may be, for example, about 20 pounds (89 Newtons). Since stopping distance directly relates to pedal force, an increase in force loss of the brake system results in an increase in stopping distance required. Due to governmental “power-off” requirements, automotive manufacturers and suppliers are scrutinizing vacuum booster force loss. Accordingly, there is a need for a vacuum booster that has lower force loss, especially during a power-off situation, than conventional vacuum boosters.